Stem cells that demonstrate tropism toward pathologic conditions of the brain are being modified to carry a variety of therapies directly to the local site of brain pathology. This natural tropism is particularly important in the brain, where site-selective delivery of therapies is problematic. For example, delivery of chemotherapy specifically to brain tumors is hampered by the blood brain barrier (BBB) and getting around it is technically challenging, prompting development of new devices and methods for intra-arterial, intranasal and stereotactic surgical delivery of chemotherapeutic drugs. All of these approaches carry an inherent risk of damage from the procedure itself. In addition, spillover of toxic therapies into normal brain tissue can have devastating results. This short review highlights a variety of approaches to modifying stem cells to carry therapeutics specifically to sites of pathology in the brain.

Neural Stem Cells

In a seminal work, Karen Aboody et al.1 demonstrated that neural stem cells (NSC) delivered to the brain or ventricles delivered past the BBB migrate through normal brain tissue to gliomas, (including glioblastomas). This study opened the door to the use of NSC as targeted delivery vehicles for malignant tumors in the brain. Glioblastoma is the most common primary malignant tumor of the brain and generally carries a dismal prognosis with traditional radiation and chemotherapeutic approaches. Thus, it is not surprising that glioblastoma has been a major focus of stem cell-based carrier therapies. The strong tropism of NSC for glioblastoma has translated into clinical trials using NSC that are genetically modified to carry cytosine deaminase, enabling conversion of 5-fluorocytosine to the chemotherapeutic agent 5-fluorouracil at the tumor site (Clinicaltrials.gov #NCT01172964).

Mesenchymal Stem Cells Also Demonstrate Tropism to Gliomas

Because NSC are relatively more difficult to obtain and expand compared to mesenchymal stem cells (MSC), considerably more work using MSC as multipurpose delivery vehicles has been reported. Use of MSC as delivery vehicles for therapies for brain pathology also takes advantage of the natural tropism of MSC to sites of inflammation, including irradiated tissue, wounds and the tumor microenvironment.2 At the molecular level, a variety of pathways have been reported to contribute to the tumor tropism of MSC including vascular cell adhesion molecule-1, transforming growth factor-beta, hepatocyte growth factor, tumor necrosis factor-alpha, platelet-derived growth factor-BB and -D, toll-like receptors and matrix metalloprotein-2 via the stromal-derived factor-1/chemokine C-X-C motif receptor-4 (SDF-1/CXCR4) axis. Tumor hypoxia is an important factor in mediating migration of NSC and MSC to gliomas, taking advantage of many of these same pathways. It is important to note that the MSC used to date in preclinical and clinical studies are heterogeneous, which may contribute to the reported heterogeneity in mechanisms of tropism. A better understanding of the mechanisms used to direct migration of MSC to tumors will certainly help to refine and enhance MSC-based delivery of therapeutics. For instance, MSC tropism to gliomas can be boosted by overexpression of CXCR4 in the cells.3

The precise MSC source may also be a critical factor for central nervous system therapy. Based on in vitro studies, concern has been raised that adipose-derived MSC can promote glioblastoma growth whereas umbilical cord blood-derived MSC inhibit glioblastoma growth.4 Furthermore, glioblastomas contain MSC-like cells that can promote tumor growth5 and the influence of these host tumor cells — via secreted signals — on the phenotype of incoming MSC from other sources is completely unknown. With any stem cell therapy, aberrant growth or differentiation under the influence of the host microenvironment remains a concern.

A Wide Variety of Therapeutic Cargos

Despite these safety concerns, in the face of devastating brain tumors — and with the safe use of MSC reported in multiple clinical studies and trials — stem cells are being redesigned to carry a wide range of therapeutic cargos to the brain. A variety of anti-tumor therapeutic transgenes have been used in preclinical studies of brain tumors including herpes simplex virus thymidine kinase, as a suicide gene; interleukins-2, -4, -12 and -23; tumor necrosis factor-related apoptosis inducing ligand (TRAIL); and interferon-beta (reviewed in 6).

Stem cells loaded with nanoparticle therapeutics are a new approach to treating gliomas. Once stem cell tropism is co-opted to gain access to the tumor, nanoparticles — generally one to 100 nm particles — loaded with drugs can be engineered to mediate sustained drug release and tumor permeability.7 Preclinical studies suggest that packaging chemotherapeutic agents in nanoparticles is an effective way to enhance the pharmacokinetic/pharmacodynamic profile of chemotherapeutic agents directed against gliomas.7

MSC infected with oncolytic viruses, such as myxoma virus, survive well — despite supporting multiple rounds of viral infection — and maintain their tropism to gliomas.8 MSC engineered to secrete immunotoxins represent another potential anti-glioma therapy. For example, MSC infected with adenoviral vectors packaged with an immunotoxin to the ephrin-A2 receptor, which is overexpressed in gliomas, can deliver the effective toxin to tumor cells in preclinical models.9

MSC also naturally secrete microvesicles and their contents can be horizontally transferred from MSC to gliomas — yet another potential route of MSC-mediated delivery of therapeutics. Microvesicles containing microRNAs and some tumor-suppressing microRNAs10 as well as antibodies to microRNAs that mediate chemotherapy resistance11 have been successfully transferred from MSC to gliomas in experimental models. These are early days for understanding the complexity and heterogeneity of microRNA in MSC microvesicles and how they can be manipulated for anti-tumor effects. Considerably more preclinical work is needed before translation of these therapies to the clinic.

Neurodegenerative Disease Targets

Brain tumors are not the only target for MSC-based delivery of therapeutic agents. For instance, Jan Nolta's group at the University of California, Davis is studying MSC delivery of brain- and glial cell line-derived neurotrophic factors and small interfering RNA in the context of neurodegenerative Huntington's disease and other trinucleotide repeat disorders.12 The growth factors are used to support survival of neurons put at risk of degeneration by toxic accumulation of pathologic proteins. The silencing RNA strategy is used to decrease levels of the toxic proteins. Multiple groups have shown proof of concept that silencing RNA strategies can decrease the accumulation and aggregation of mutant, neurotoxic proteins and prolong survival in animal models of these neurodegenerative diseases.

Stroke

Stem cells genetically modified to overexpress therapeutic genes also have been studied in animal models of stroke. This approach relies on the ability of stem cells to migrate toward a hypoxic environment. The genetic cargo in these models includes a variety of growth factor genes delivered by MSC and NSC as well as the antioxidant gene, Cu/Zn superoxide dismutase, delivered by NSC.13

Summary

NSC and MSC are now in clinical trials as potential therapies for a variety of nervous system diseases. Major safety issues using unmodified versions of these stem cells have not emerged in regulated clinical trials. Some modified versions of these cells also have been studied in clinical trials without major safety issues arising. Given the unmet medical need represented by limited treatments for malignant gliomas and other deadly neurologic diseases, delivery of novel therapeutics by stem cells that traffic to sites of brain pathology is a rational extension of the early stem cell trials to treat diseases of the brain and one of the more exciting frontiers in cell therapies.